imap: Fix potential hang/crash when unhibernating

imap: Fix potential hang/crash when unhibernating

This fixes two potential bugs:

a) Unhibernation is triggered by new mailbox changes. While sending these
to IMAP client, Dovecot notices that the client has disconnected. The
process will stay IDLEing for 30 minutes until it gets disconnected with
"Disconnected: Inactivity - no input for 1800 secs"

b) Unhibernation is triggered by DONE command with some further pipelined
commands. During unhibernation new mailbox changes are noticed, and
again IMAP client is found to be disconnected while sending the changes.
This causes a segfault.